Two commonly used passive microwave devices for combining RF energy from different bands of wavelengths are a waveguide directional coupler and a turnstile combiner.
FIG. 1 shows the waveguide directional coupler, wherein two side-by-side waveguides 1 and 2 are coupled together using apertures 3. RF energy in a first band of wavelengths (f1) enters through Port.sub.-- 1 and RF energy in a second band of wavelengths (f2) enters through Port.sub.-- 2. In this example it is assumed that f2 is higher in frequency than f1 and, as a result, waveguide 2 has smaller dimensions than waveguide 1. The RF energy from the smaller waveguide 2 couples through the apertures 3 into the larger waveguide 1, but not vice versa. The end result is that the RF energy is combined, and both f1 and f2 exit through Port.sub.-- 3, with minimal f2 energy exiting through Port.sub.-- 4. An f1 matching iris (not shown) may be placed at Port.sub.-- 1 to mitigate the effect of apertures 3, which may cause some of the f1 energy to reflect back to Port.sub.-- 1.
FIG. 2 illustrates the conventional turnstile combiner. In this device a centrally placed waveguide 4 which supports energy in the band of wavelengths f1 defines the Port.sub.-- 1 and Port.sub.-- 3, as in FIG. 1. However, Port.sub.-- 2 and Port.sub.-- 4 are implemented as four symmetrically placed waveguides 5A-5D that feed energy in the band of wavelengths f2 into the central waveguide 4. In practice, the waveguides 5A-5D may be folded against the central waveguide 4. The central waveguide 4 may be a square waveguide, as shown, or could be a coaxial waveguide, or a circular waveguide. The RF energy for f2 enters through either the vertical pair of rectangular ports (Port.sub.-- 2) or through the horizontal pair of rectangular ports (Port.sub.-- 4), and exits through Port.sub.-- 3. Each pair of f2 ports can be viewed as a single port since a pair must be excited at a time to create the desired mode. As in the embodiment of FIG. 1, the RF energy for f2 is coupled into the larger waveguide 4 through holes or apertures 6.
General reference in this regard can be had by referring to the following two publications for showing a C/Ku dual band diplexer and a 4, 6 and 11 GHz combiner, respectively: M. Iida et al., "13 Meter C/Ku Dual Frequency Band Earth Station Antenna", NEC Res. & Develop., vol. 99, pp. 98-112, 1990; and E. T. Harkness, "A Network for Combining Radio Systems at 4, 6 and 11 kmc", The Bell System Tech. Journal, September 1959, pp. 1253-1267.
A problem is created when two frequency bands of interest are close to one another, such as when one wishes to use the X-band (7.25 GHz to 8.4 GHz) and the Ku-band (10.95 GHz to 14.5 GHz). The problem in a broadest aspect relates to providing a single passive combiner/divider having the necessary bandwidth to support operation in both bands, without introducing higher order modes that would adversely affect the operation of the upper band. The conventional aperture-coupled combiners that are known to the inventors do not exhibit the necessary bandwidth for this type of dual band operation.